Method and apparatus for electro-optic delay generation of optical signals

ABSTRACT

An optical delay generator includes a waveguide made from electro-optically active material which contains a chirped distributed Bragg reflector. An electric field generated across the waveguide causes the index of refraction within the waveguide to change. A change in the index of refraction results in a change in the point at which light is reflected from the chirped distributed Bragg reflector within the waveguide, thus providing a controllable delay for optical pulses. Optical pulse position modulation is provided by using the optical delay generator to control the delay imparted on each pulse within a stream of equally-spaced optical pulses.

FIELD OF THE INVENTION

[0001] The present invention relates to the processing of opticalsignals and, more particularly, to delaying optical signals.

BACKGROUND OF THE INVENTION

[0002] Many satellite and terrestrial optical communication systemsrequire transmission of analog optical signals. One mechanism for thetransmission of analog optical signals is through the use of some sortof pulse modulation, where a stream of optical pulses is modulated by ananalog signal. Pulse Position Modulation (PPM) is a well-knownmodulation technique for radio-frequency transmissions. It is also usedin analog optical communications. In PPM, a shift in the position ofeach pulse represents a sample of the original analog signal. Since thepulse repetition frequency (PRF) of the optical pulses must be greaterthan twice the bandwidth of the analog signal to correctly sample theanalog signal, PRFs for optical communications will be quite high. Forexample, an optical inter-satellite link designed to transmit waveformswith a bandwidth of 20 GHz requires a PRF of over 40 GHz.

[0003] The optical pulses within the stream should be of short duration,since it is well known in the art that PPM signal-to-noise ratio (SNR)performance improves as the pulse widths within the modulated pulsestream decrease. Pulse widths as short as 0.3 picoseconds may bedesirable for a PPM optical communication system. However, is also wellknown in the art that PPM performance will suffer if the shapes of theoptical pulses vary or the amplitudes of the pulses vary on apulse-to-pulse basis. Mode locking of a pulsed laser is a maturetechnique for producing equally spaced ultra-short identical pulses. Itwould be beneficial to use a mode-locked laser in a PPM communicationsystem if the equally-spaced pulses produced by the system could bemodulated without distortion.

[0004] Therefore, implementations of PPM for optical communicationsrequire a mechanism for modulating the delays between extremely shortoptical pulses within a pulse stream without modulating the shapes orpulse-to-pulse amplitudes of the pulses. Direct modulation of asemiconductor laser will appropriately modulate the delay between theoptical pulses generated by the laser. However, a directly modulatedsemiconductor laser generates relatively long pulses that result inlimited SNR performance. Pulse compression can be used on the longerpulses produced by the directly modulated semiconductor laser, butdevices to provide such compression are complex and cumbersome. Directmodulation of a semiconductor laser may also introduce amplitudemodulation or pulse reshaping of the individual time-shifted pulses,further limiting performance.

[0005] Pulse position modulation of extremely short optical pulses isalso achieved by applying a pulse-to-pulse delay external to the sourceof the equally spaced optical pulses. That is, a method and apparatusare used that can receive a stream of optical pulses, change thepulse-to-pulse delay at the rate required for properly sampling thetransmitted analog signal, and further transmit the delayed pulses. Itis known in the art that one example of a pulse position modulator foroptical pulses consists of an optical delay line, such as a parallelslab of transparent electro-optically active material. The refractiveindex of the electro-optically active material can be controllablyvaried by an applied voltage, so that each pulse is controllably delayedupon traversing the electro-optically active material in accordance withthe instaneous voltage. However, such a modulator requires anundesirably large amount of electrical power, due to the relativelylarge voltages required to modulate the refractive index of the materialand thus modulate the delay encountered by a pulse traversing thematerial.

[0006] Another example of a pulse position optical modulator relyingupon the use of electro-optically active material is disclosed in U.S.Pat. No. 3,961,841, issued Jun. 8, 1976 to Giordmaine. Giordmainediscloses a device for optical pulse position modulation comprising adiffraction grating in combination with an electro-optic prism and alens. The diffraction grating splits an incident light pulse into itsfrequency components and the lens directs the components into the prism.The refractive index change provided by the prism causes a phase shiftin the frequency components and thus a time shift in the optical pulseonce it is reconstructed by the diffraction grating. The devicedisclosed by Giordmaine provides the capability of modulating lightpulses as short as one picosecond. However, the maximum controllabledelay is limited to a few picoseconds for a 3 picosecond pulse andfurther decreases for shorter pulses. Also, the multiplicity of opticalelements such as the diffraction grating, lens, and prism increase thecomplexity and manufacturing cost of the device.

[0007] A device for delaying optical pulses is disclosed in U.S. Pat.No. 5,751,466, issued May 12, 1998 to Dowling et al and is shown inFIG. 1. Dowling discloses a photonic bandgap structure comprising aplurality of cells 18A-18N of width d in which the refractive indexvaries. The refractive index variation may be such that each cellcomprises two layers of materials with two different indices ofrefraction n₁ and n₂. If the widths of the two layers within each cellare λ/4n₁ and λ/4n₂ where λ is the free space wavelength of the opticalpulse to be delayed, a distributed Bragg reflector structure is created.According to Dowling, the thickness and/or number of layers in thephotonic bandgap structure and/or their indices of refraction areselected to produce a structure with a transmission resonance centerfrequency and bandwidth corresponding to the frequency and bandwidth ofthe optical pulse to be delayed. By matching the transmission resonanceto the optical pulse, a controllable delay is imparted to the opticalpulse without significantly altering the optical signal.

[0008] The device disclosed by Dowling requires that the thickness ofeach layer in the device be approximately one-half the wavelength of theincident optical pulse to form the photonic bandgap structure. The delayimparted on an optical signal by transmission through the structure willdepend upon the number of layers and the indices of refraction withinthe layers. The structure can be thought of as essentially increasingthe length of the waveguide in which it is contained, thus providing thedesired delay. For example, Dowling discloses a simulation of a photonicbandgap structure that is 7 μm thick that provides a delay equivalent toan optical signal traveling through a 110 μm stucture, or a delay ofabout 0.4 picoseconds. Since the amount of delay from a single structureis relatively small, Dowling discloses that the structures can besuccessively coupled in a single device to provide additional delay. Ofcourse, this increases the overall size of the device.

[0009] Dowling also discloses that the delay provided by a photonicbandgap structure can be varied by changing the indices of refractionwithin the layers of the structure. One way to accomplish this is tofabricate at least one of the layers from electro-optically activematerial. An applied voltage will then change the index of refraction inthe layer to which the voltage is applied. FIG. 1 shows a voltage means15 that applies a voltage to one or more of the layers within the devicedisclosed by Dowling. Varying the voltage would vary the delay, thusproviding the controllable delay required for pulse position modulation.However, since the overall delay provided by photonic bandgap structureis relatively small, it would follow that the change of delay providedby electro-optically changing the indices of refraction would only besome fraction, typically 0.1% or less, of that relatively small delay.Again, this limitation could be overcome by coupling successivestructures, with a corresponding increase in the overall size of thestructure.

[0010] There exists a need for a high quality optical delay apparatusand method that provide large, controllable delays for short opticalpulses. Moreover, the apparatus and method must be capable of providingthe required delay without substantially altering the pulse-to-pulseamplitude or shape of the pulses in the original pulse stream.Additionally, it is important for the delay generation to be implementedin a compact, lightweight apparatus that is compatible with otherintegrated systems.

SUMMARY OF THE INVENTION

[0011] Accordingly, it is an object of the present invention to provideapparatus and methods for optical delay generation.

[0012] It is another object of the present invention to provideapparatus and methods for optical delay generation without causingpulse-to-pulse amplitude modulation or pulse reshaping of delayedoptical pulses.

[0013] It is another object of the invention that the method andapparatus provide optical delay that can be used for pulse positionmodulation.

[0014] These and other objects are provided according to the presentinvention by transmitting optical pulses to be delayed into a waveguidemeans comprising electro-optically active material within which isformed a chirped distributed Bragg reflector (C-DBR) oriented in thedirection of light propagation within the waveguide means. The chirpeddistributed Bragg reflector reflects light at different wavelengths atdifferent points within the waveguide. An electric field generatorgenerates and controls an electric field applied across the waveguide ina direction perpendicular to the direction of propagation. Changes inthe electric field intensity cause changes in the index of refractionwithin the waveguide means, thus changing the point at which the opticalpulses reflect from the chirped distributed Bragg reflector and aretransmitted out of the waveguide means. Thus, optical delay generationis accomplished by controlling the intensity of the electric fieldacross the chirped distributed Bragg reflector.

[0015] In a first specific embodiment of the present invention, thewaveguide means comprises a straight waveguide constructed fromelectro-optically active material, such as lithium niobate, sandwichedbetween a top conductor and a bottom conductor. A chirped distributedBragg reflector is formed in the waveguide by quasiperiodicallycorrugating the waveguide walls. A voltage source is connected to thetop conductor and the bottom conductor such that a voltage between thetwo is created. The voltage causes an electric field to be generatedacross the chirped distributed Bragg reflector, thus changing the indexof refraction as the voltage changes. An alternate embodiment uses atapered waveguide in which the waveguide walls are periodicallycorrugated.

[0016] In a second embodiment of the present invention, theelectro-optically active material used in the waveguide means comprisesa semiconductor chirped distributed Bragg reflector structure withexcitonic band just above the photon energy structure. The chirpeddistributed Bragg reflector is formed by the individual layers ofsemiconductor material. The refractive index and thickness of each layervary from its neighbor so as to provide the quasiperiodic variation inrefractive index required to form a chirped distributed Bragg reflector.

[0017] The chirped distributed Bragg reflector of an alternativeembodiment of the present invention comprises an apodized chirpeddistributed Bragg reflector. Apodization of the chirped distributedBragg reflector reduces the oscillations in the group delay of theoptical pulse that would result if the optical pulse were reflected by alinearly chirped distributed Bragg reflector. Hence, distortion ofoptical pulses is reduced.

[0018] Reflection of optical pulses from a chirped distributed Braggreflector results in broadening of the optical pulses due to an acquiredchirp. Therefore, in another embodiment of the present invention, thetime-delayed pulses output from the delay generator are passed through adispersion compensating fiber, which provides correction for theacquired chirp.

[0019] The present invention is used to provide optical pulse positionmodulation for an analog signal. A stream of equally-spaced opticalpulses is transmitted into a waveguide containing a chirped distributedBragg reflector. The analog signal controls a modulation means thatgenerates an electric field across the waveguide. The modulation meanscontrols the intensity of the electric field and thus the delay providedby the waveguide. Each optical pulse in the stream of optical pulses isreflected by the chirped distributed Bragg reflector and acquires adelay corresponding to the analog signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 (prior art) shows a photonic bandgap structure used fordelaying optical pulses.

[0021]FIG. 2 shows a schematic representation of an optical delaygenerator in accordance with the present invention comprising awaveguide with a chirped distributed Bragg reflector and an opticalcirculator for directing pulses into and out of the waveguide.

[0022]FIG. 3A shows a graphical representation of the variation inrefractive index required for a chirped distributed Bragg reflector.

[0023]FIG. 3B shows a graphical representation of the variation inrefractive index required for an apodized chirped distributed Braggreflector.

[0024]FIG. 4 shows an alternative embodiment of the present inventioncomprising a straight waveguide where the core width variesquasiperiodically to create a chirped distributed Bragg reflector.

[0025]FIG. 5 shows another embodiment of the present inventioncomprising a tapered waveguide where the core width varies periodicallyto create a chirped distributed Bragg reflector.

[0026]FIG. 6 shows another embodiment of the present inventioncomprising a waveguide with a plurality of electrodes where variationsin the electric field generated by the separate electrodes create achirped distributed Bragg reflector.

[0027]FIG. 7 shows an embodiment of the present invention comprising asemiconductor chirped distributed Bragg reflector structure where layersof varying width and refractive index form the chirped distributed Braggreflector.

[0028]FIG. 8 shows another embodiment of the present invention where anoptical signal reflected from a chirped distributed Bragg reflector isdirected into a dispersion compensating filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. In the drawings, the thicknesses oflayers and regions are exaggerated for clarity.

[0030] Referring now to FIG. 2, a schematic representation of apparatusand methods for optical delay generation is shown. Referring to FIG. 2,a waveguide means 100 receives light pulses 31 a at times t_(n) andimparts a delay Δt_(n) on each pulse to thereby produce reflected lightpulses 31 b at times t_(n)+Δt_(n). Preferably, light pulses 31 b aresimply delayed versions of light pulses 31 a such that the delayedpulses 31 b replicate the original light pulses 31 a in terms of pulseshape and have not acquired any pulse-to-pulse amplitude modulation.

[0031] Still referring to FIG. 2, the waveguide means 100 comprises oneor more layers of electro-optically active material 107 with a varyingrefractive index. The present invention requires that the variations inthe refractive index within the electro-optically active material form achirped distributed Bragg reflector (C-DBR). The C-DBR reflects anoptical signal of a specific wavelength after the optical signal hastraveled a certain distance z within the waveguide means, as indicatedby line 108. The construction and features of the C-DBR will bedescribed in more detail below.

[0032] The index of refraction of electro-optically active materialchanges when an electrical field is applied. In FIG. 2, a means forgenerating an electric field across the electro-optically activematerial 107 is shown as being provided by an upper electrode 105 and alower electrode 106 connected to a voltage source 109. As the electricfield between the two electrodes changes, the index of refraction withinthe electro-optically active material will change. The change of therefractive index is generally proportional to the magnitude of theelectric field and is represented by dn=(dn/dE)dE. As will be describedbelow, changes in the refractive index result in changes in the distancein which an optical signal travels in the C-DBR before it is reflected,resulting in changes in the amount of delay applied to an opticalsignal.

[0033] Still referring to FIG. 2, the optical delay generator apparatusalso includes means for directional coupling the optical signals intoand out of the waveguide means A fiber optic circulator 30 or otherconventional coupling means may be used. Ideally, the circulator orother coupling means is low loss and will not cause any pulse reshapingor pulse-to-pulse amplitude modulation.

[0034] Electro-optically active materials are well known in the art.However, for use in the present invention, the electro-optically activematerial must be such that a C-DBR can be formed within it. Also, thedn/dE factor should be as large as possible, so that the magnitude ofthe electric field can be kept as small as possible. Materials thatprovide such characteristics include lithium niobate (LiNbO₃), lithiumtantalate (LiTaO₃), and lithium niobate doped with titanium. Theelectro-optically active material may also comprise anelectro-refractive semiconductor C-DBR structure. A semiconductor C-DBRstructure comprises several very thin layers of materials havingdifferent refractive indices. Each layer comprises semiconductormaterial known to exhibit an excitonic band just above the photonenergy. With such material, the electrical field magnitude required toproduce the desired refractive index change is reduced.

[0035] A periodic (or quasiperiodic) fluctuation in the core refractiveindex of optical media results in a “Bragg grating” or a “distributedBragg reflector.” The pattern of fluctuations behaves as a spectrallyselective reflector for electromagnetic radiation. The reflection of adistributed Bragg reflector reaches its maximum at the wavelength λsatisfying the Bragg condition:

β(λ)=π/Λ  (1)

[0036] where β(λ) is the wave number at the given wavelength and Λ isthe period of modulation of the distributed Bragg reflector.

[0037] The present invention requires that the distributed Braggreflector be quasiperiodic instead of periodic. That is, the period ofthe refractive index variation (i.e., the linear distance, betweensuccessive peaks and valleys of the refractive index profile) is not aconstant, but instead changes in a predetermined fashion along thepropagation axis of the distributed Bragg reflector. The propagationaxis of the DBR is the direction in which the incident light travels.Such a Bragg reflector is referred to as a “chirped” distributed Braggreflector. Preferably, the present invention utilizes a quasiperiodicvariation in the refractive index in which the period increases ordecreases as an approximately linear function of position along thepropagation axis, resulting in a linearly chirped distributed Braggreflector. FIG. 3A shows a “chirped” variation of the refractive index nas a function of position z along the propagation axis.

[0038] As indicated above, a chirped distributed Bragg reflector iscreated in electro-optically active material by modulating therefractive index within the material. Creation of chirped distributedBragg reflectors is well known in the art. U.S. Pat. No. 4,953,939,issued Sep. 4, 1990 to R. Epworth, describes several methods forcreating chirped distributed Bragg reflectors within optical fibers. Allof the methods disclosed by Epworth describe the creation ofquasi-periodic corrugations within the walls of the optical fiber, wherethe wall of the fiber is the interface between a core and a claddingwithin the fiber. These methods for creating a chirped distributed Braggreflector within optical fibers would also be used to create chirpeddistributed Bragg reflectors within straight waveguides made fromelectro-optically active material such as lithium niobate or lithiumtantalate as used in some embodiments of the present invention. It isalso known in the art that a chirped distributed Bragg reflector willresult when a tapered waveguide contains periodic corrugations on itswalls, where the period of the corrugations roughly corresponds to theBragg wavelength.

[0039]FIG. 4 illustrates an embodiment of the present invention whichuses a straight waveguide. In FIG. 4, a straight waveguide 120 has acore 124 and a cladding 125. The waveguide 120 has been constructed suchthat width of the core 125 varies in a quasiperiodic fashion so as tocreate a chirped distributed Bragg reflector. The waveguide 120 issandwiched by a top electrode 123 and a bottom electrode 122, which areconnected to a voltage source 129 to generate a voltage and thus anelectric field across the waveguide. Light pulses 31 a enter one end ofthe waveguide 120 and are reflected by the chirped distributed Braggreflector within the waveguide 120. An alternative embodiment is shownin FIG. 5, where a tapered waveguide 130 with a core 134 and a cladding135 is constructed such that the width of the core 134 varies in aperiodic fashion.

[0040] A method known in the art as polling also creates a chirpeddistributed Bragg reflector. In polling, a quasi-periodic DC electricfield is applied along a waveguide constructed from electro-opticallyactive material. The quasi-periodic variations in the DC electric fieldcause quasi-periodic variations in the local index of refraction,resulting in a chirped distributed Bragg reflector. FIG. 6 illustratesan embodiment of the present invention that uses polling. A waveguide140 contains electro-optically active material 144 sandwiched between aplurality of top electrodes 142 and a bottom conductor 143. A voltagesource 146 connects to the plurality of top electrodes 142 and thebottom conductor 143 so as to provide an electric field between each topelectrode 142 and the bottom conductor 143. The voltage source 146controls the voltage at each electrode so as to provide a voltage thatis the sum of a two voltage components as shown below:

V _(i) =V _(uniform) +V _(poll)(i)

[0041] where V_(i) is the total applied voltage at the ith electrode,V_(uniform) is a voltage to be applied uniformly across the waveguideand V_(poll)(i) is a voltage to be applied at the ith electrode toachieve polling. The first voltage component provides a uniform electricfield that would result in a uniform change in the index of refractionalong the waveguide 140 in the absence of the second voltage component.Thus, the first voltage component controls the delay provided by thewaveguide. The second voltage component provides an electric field thatvaries quasi-periodically along the length of the waveguide so as toresult in a quasi-periodic variation in the index of refraction withinthe waveguide 140 in the absence of the first voltage component. Thus,the second voltage component creates a chirped distributed Braggreflector within the waveguide.

[0042] If a semiconductor C-DBR structure is used to provide thewaveguide means of the present invention, the chirped distributed Braggreflector is formed by controlling the refractive index within theindividual layers of the semiconductor structure. One such semiconductorstructure can be formed from alternating layers of low refractive indexaluminum arsenide and high refractive index aluminum gallium arsenide.FIG. 7. shows an embodiment of the present invention using asemiconductor structure with alternating low and high refractive indexlayers. In FIG. 7, alternating layers of low refractive index material(151A . . . 151N) and high refractive index material (152A . . . 152N)are used to form an electro-optically active waveguide. Each alternationbetween a low index layer and high index layer is a single refractiveindex period. The thickness of the alternating layers (d_(A) . . .d_(N)) is increased in a quasiperiodic fashion to provide a linearlyincreasing change in each refractive index period. A uniform electricfield, controlled by a voltage source 159 and applied across thestructure by a first electrode 153 and a second electrode 154, controlsthe amount of delay provided by the structure.

[0043] A chirped distributed Bragg reflector can also be formed within asingle layer of electro-optically active material by controlling thedoping of that layer of material. For example, the refractive index of alithium niobate waveguide can be modulated by doping the lithium niobateperiodically or quasiperiodically with titanium. A chirped distributedBragg reflector will result if the periodicity of the doping satisfiesthe Bragg condition described above.

[0044] To further describe the apparatus and method of the presentinvention, reference is made to the straight waveguide embodiment of thepresent invention, as shown in FIG. 4. In this case, the wave numbersatisfying the Bragg condition is approximately given by β=2π/nλ, wheren is the effective refraction index. In a linearly chirped DBR,

Λ(z)=Λ₀Λ′z  (3)

[0045] where Λ′ is the chirp parameter, and z is the classical turningpoint for a given wavelength. The chirp parameter Λ′=ΔΛ/L where ΔΛ isthe change in the C-DBR period across a C-DBR structure of total lengthL. The turning point z is found from the Bragg condition equation:

nλ=2π/β=2π/(π/Λ(z))=2(Λ₀ +Λ′z)  (4)

[0046] The group delay introduced by a chirped distributed Braggreflector is approximately

t _(d)=2nz/c  (5)

[0047] where c is the speed of light. This equation demonstrates thatthe group delay can be changed by changing the effective refractiveindex of the waveguide. If the index of refraction is changed as aresult of the application of an electric field to an electro-opticallyactive material, the group delay will be changed as shown in theequation below:

dt _(d)=(2dn/cΛ′)(nλ−Λ ₀)≈2(Λ₀ /cΛ′)dn  (6)

[0048] The change in the index of refraction will be nearlyinstantaneous to the change in the electric field, thus providing thecapability to quickly change the delay provided by the presentinvention. Changes in the electric field, however, will be limited bythe means used to apply and control the electric field. Controlelectronics, impedance of electrodes, and other electrical effects maylimit the speed at which the present invention operates, but the presentinvention itself can provide nearly instantaneous change in applieddelay. Electro-optical modulators that do not use a chirped distributedBragg reflector have been demonstrated to operate at 50 GHz, so thepresent invention is also expected to provide operation up to 50 GHz orhigher.

[0049] The classical description of group delay presented above is onlyan approximation. For example, it does not account for oscillations inthe group delay that result from a linearly chirped grating. However, acorrectly engineered chirp will eliminate the resonances and a lineargroup delay dependence is realized. One such chirped grating that willreduce the oscillations is an apodized chirped distributed Braggreflector. In an apodized chirped distributed Bragg reflector, theamplitude of the chirped index of refraction is tapered from a minimumto a maximum and then back to a minimum within the chirped distributedBragg reflector. FIG. 3B illustrates the refractive index variation foran apodized chirped distributed Bragg reflector.

[0050] Proper choice of the length and chirp of the C-DBR provides thedesired delay range for optical pulses of a certain duration, and thusprovides the capability for pulse position modulation. To illustrate thecalculations used, a delay generator based on a straight waveguide witha linearly varying C-DBR period is used, as shown in FIG. 4. Theelectro-optically active material used in the waveguide is LiNbO₃,although, as indicated previously, other materials may be used. Anyunwanted resonances in the group delay are eliminated by the procedurepreviously described.

[0051] The average C-DBR period is determined by the wavelength of theoptical source and is calculated from the equation for the Braggcondition. For an optical wavelength λ=1.55 mm and the LiNbO₃ index ofrefraction n_(e)=2.2, the average C-DBR period Λ₀=n_(e)λ/2=1.75 mm. Therequired differential group delay dt_(p) determines the chirp for agiven differential index of refraction dn. It is well known in the artthat for LiNbO₃, the differential index of refraction as a function ofthe applied electric field is derived from the equation:

dn=r₃₃n_(e) ³dE_(z)  (7)

[0052] where the electro-optic coefficient r₃₃=30.8×10⁻¹² m/V. Assuminga 3 mm-wide waveguide and 5 V of applied voltage, the differential indexof refraction dn=5.5×10⁻⁴. If a group delay dt_(p)=5 ps is required, thechirp of the C-DBR Λ′=10⁻² μm/cm.

[0053] The delay generator must have sufficient bandwidth to reflectshort optical pulses. It is well known in the art that the approximatespectral width of a chirped distributed Bragg reflector

Δλ_(C-DBR)=2Λ′L/n_(e),  (8)

[0054] where L is the total length of the distributed Bragg reflector.To reflect the full spectrum of the optical pulses transmitted into thedelay generator containing the distributed Bragg reflector, the lengthof the reflector L must be greater than Δλn_(e)/2Λ′. For example, thespectral width of a Gaussian pulse of t=0.3 ps duration and wavelengthλ=1.55 mm is Δλ_(t)=0.44λ²/ct=12 nm. For the delay generator usingLiNbO₃ previously described, the length of the DBR must exceedL=Δλ_(t)n_(e)/2Λ′≈1.3 cm. Electro-optical waveguide modulators with L=1cm and longer are common in the art.

[0055] The present invention provides optical pulse position modulationby using a modulation means to control the optical delay provided by thechirped distributed Bragg reflector as shown in FIG. 2. For pulseposition modulation, a stream of equally spaced optical pulses istransmitted into the input of an optical circulator 30. The opticalcirculator than sends these pulses into the waveguide 100 comprisingelectro-optically active material and containing a chirped distributedBragg reflector. An analog signal to be pulse positioned modulatedcontrols the voltage source 109. The voltage source 109 controls anelectric field within the waveguide 100, and thus controls the delayimparted on each optical pulse 31 a transmitted into the waveguide 100.Each optical pulse 31 a will acquire a delay corresponding to the delayrequired to pulse position modulate the analog signal, and will bereflected out of the waveguide 100 and transmitted out of the circulator30 as a delayed pulse 31 b.

[0056] The optical pulses reflected by the described delay generator arebroadened due to the chirp imposed by the C-DBR. This broadening or“chirping” of the pulses may reduce the performance of a pulse positionmodulation system provided by the present invention. However, this chirpcan be removed by passing the chirped pulses through a dispersioncompensating fiber as is customary in the art. FIG. 8 demonstrates oneway in which the acquired chirp can be removed. In FIG. 8, the delayedoptical pulses output by the coupler 31 b are sent through a dispersioncompensating fiber 32. Dispersion compensating filters are well known inthe art. The filtered pulses 31 c output by the dispersion compensatingfilter 32 should match the optical pulses 31 a input to the system inamplitude and pulse width.

[0057] Pulse position modulation provided by the present inventionminimizes pulse-to-pulse amplitude or shape modulation. As shown in theexample previously described, the change in the refraction index,dn=5.5×10⁻⁴<<n_(e)=2.2 and the accompanying shift of the C-DBR band toachieve the desired group delay dt_(d)=5 ps are very small, that isdλ/dλ_(t)∝dn/n_(e)<<1. Hence, attenuation of the optical signal due tothe spectral changes in frequency response of the waveguide resultingfrom the changes in the refraction index should be negligible.

[0058] From the foregoing description, it will be apparent that thepresent invention has a number of advantages, some of which have beendescribed above, and others of which are inherent in the embodiments ofthe invention described above. Also, it will be understood thatmodifications can be made to the optical delay generator and method forperforming pulse position modulation described above without departingfrom the teachings of subject matter described herein As such, theinvention is not to be limited to the described embodiments except asrequired by the appended claims.

What is claimed is:
 1. An optical delay generator comprising: awaveguide comprising at least one layer of electro-optically activematerial; a chirped distributed Bragg reflector formed within said atleast one layer of electro-optically active material, said chirpeddistributed Bragg reflector having a direction of propagation; and, ameans for applying an electric field across said electro-opticallyactive material, said electric field applied in a direction normal tosaid direction of propagation of said chirped distributed Braggreflector.
 2. The optical delay generator of claim 1 wherein saidwaveguide comprises: a cladding, and a core within said cladding, saidcore having walls with corrugations to form said chirped distributedBragg reflector
 3. The optical delay generator of claim 2 wherein saidwaveguide is tapered and said walls have periodic corrugations.
 4. Theoptical delay generator of claim 2 wherein said waveguide is of constantwidth and said walls have quasiperiodic corrugations.
 5. The opticaldelay generator of claim 1 wherein said means for applying an electricfield applies an electric field that varies in intensityquasiperiodically in a direction parallel to said direction ofpropagation of said chirped distributed Bragg reflector, whereinvariations in said electric field create said chirped distributed Braggreflector.
 6. The optical delay generator of claim 1 wherein saidwaveguide comprises: a plurality of layers of near-resonantsemiconductor material, said layers forming said chirped distributedBragg reflector.
 7. The optical delay generator of claim 1 wherein saidchirped distributed Bragg reflector has a length sufficient to reflect afull spectrum of optical pulses transmitted into said waveguide means.8. The optical delay generator of claim 1 wherein said chirpeddistributed Bragg reflector is an apodized chirped distributed Braggreflector.
 9. The optical delay generator of claim 1 wherein said meansfor inducing an electric field comprises: a first electrode located onone side of said at least one layer of electro-optically activematerial; a second electrode located on a side of said at least onelayer of electro-optically active material opposite the location of saidfirst electrode; and, a voltage source, said voltage source connected tosaid first electrode and said second electrode, wherein said voltagesource generates a voltage between said first electrode and said secondelectrode.
 10. The optical delay generator of claim 1 wherein said meansfor inducing an electric field comprises: a common electrode located onone side of said at least one layer of electro-optically activematerial; a plurality of electrodes located on a side of said at leastone layer of electro-optically active material opposite the location ofsaid common electrode; and, a voltage source, said voltage sourceconnected to said common electrode and said plurality of electrodeswherein said voltage source generates a voltage between said commonelectrode and said plurality of electrodes.
 11. The optical delaygenerator of claim 5 wherein said means for inducing an electric fieldcomprises: a common electrode located on one side of said at least onelayer of electro-optically active material; a plurality of electrodeslocated on a side of said at least one layer of electro-optically activematerial opposite the location of said common electrode; a voltagesource, said voltage source connected to said common electrode and saidplurality of electrodes wherein said voltage source generates acontrollable voltage between said common electrode and each electrode insaid plurality of electrodes, wherein said controllable voltagecomprises a first voltage uniformly applied at each electrode and asecond voltage that varies with each electrode in said plurality ofelectrodes, said second voltage creating said variations in saidelectric field.
 12. The optical delay generator of claim 1 wherein saidelectro-optically active material is lithium niobate, lithium tantalate,lithium niobate doped with titanium, aluminum arsenide, or aluminumgallium arsenide.
 13. An optical delay generator comprising: a waveguidecomprising at least one layer of electro-optically active material; achirped distributed Bragg reflector formed within said at least onelayer of electro-optically active material, said chirped distributedBragg reflector having a direction of propagation; and, a voltage sourcewhich induces an electric field across said electro-optically activematerial in a direction normal to said direction of propagation of saidchirped distributed Bragg reflector.
 14. The optical delay generator ofclaim 13 wherein said waveguide comprises: a cladding, and a core withinsaid cladding, said core having walls with corrugations to form saidchirped distributed Bragg reflector
 15. The optical delay generator ofclaim 14 wherein said waveguide is tapered and said walls have periodiccorrugations.
 16. The optical delay generator of claim 14 wherein saidwaveguide is of constant width and said walls have quasiperiodiccorrugations.
 17. The optical delay generator of claim 13 wherein saidvoltage source induces an electric field that varies in intensityquasiperiodically in a direction parallel to said direction ofpropagation of said chirped distributed Bragg reflector, whereinvariations in said electric field create said chirped distributed Braggreflector.
 18. The optical delay generator of claim 13 wherein saidwaveguide comprises: a plurality of layers of near-resonantsemiconductor material, said layers forming said chirped distributedBragg reflector.
 19. The optical delay generator of claim 13 whereinsaid chirped distributed Bragg reflector has a length sufficient toreflect a full spectrum of optical pulses transmitted into saidwaveguide means.
 20. The optical delay generator of claim 13 whereinsaid chirped distributed Bragg reflector is an apodized chirpeddistributed Bragg reflector.
 21. The optical delay generator of claim 13wherein said voltage generator comprises: a first electrode located onone side of said at least one layer of electro-optically activematerial; a second electrode located on a side of said at least onelayer of electro-optically active material opposite the location of saidfirst electrode; and, a voltage source, said voltage source connected tosaid first electrode and said second electrode, wherein said voltagesource generates a voltage between said first electrode and said secondelectrode.
 22. The optical delay generator of claim 13 wherein saidvoltage generator comprises: a common electrode located on one side ofsaid at least one layer of electro-optically active material; aplurality of electrodes located on a side of said at least one layer ofelectro-optically active material opposite the location of said commonelectrode; and, a voltage source, said voltage source connected to saidcommon electrode and said plurality of electrodes wherein said voltagegeneration means generates a voltage between said common electrode andsaid plurality of electrodes.
 23. The optical delay generator of claim17 wherein said voltage source comprises: a common electrode located onone side of said at least one layer of electro-optically activematerial; a plurality of electrodes located on a side of said at leastone layer of electro-optically active material opposite the location ofsaid common electrode; a voltage source, said source connected to saidcommon electrode and said plurality of electrodes wherein said sourcegenerates a controllable voltage between said common electrode and eachelectrode in said plurality of electrodes, wherein said controllablevoltage comprises a first voltage uniformly applied at each electrodeand a second voltage that varies with each electrode in said pluralityof electrodes, said second voltage creating said variations in saidelectric field.
 24. The optical delay generator of claim 13 wherein saidelectro-optically active material is lithium niobate, lithium tantalate,lithium niobate doped with titanium, aluminum arsenide, or aluminumgallium arsenide.
 25. A method for delaying optical pulses comprisingthe steps of: coupling said optical pulses into a waveguide comprisingelectro-optically active material, said waveguide having a chirpeddistributed Bragg reflector formed within said electro-optically activematerial that reflects said optical pulses at a turning point withinsaid waveguide and said electro-optically active material having anindex of refraction; applying an electric field to saidelectro-optically active material to change said index of refraction ofsaid electro-optically active material so as to change said turningpoint of said chirped distributed Bragg reflector, the change of turningpoint proportional to the amount of desired pulse delay; and, couplingsaid optical pulses reflected from said chirped distributed Braggreflector out of said waveguide.
 26. The method of claim 25 wherein saidchirped distributed Bragg reflector is an apodized chirped distributedBragg reflector.
 27. The method of claim 25 further comprising the stepof: transmitting said optical pulses coupled out of said waveguide intoa dispersion compensating filter.
 28. An optical delay generatorcomprising: a waveguide made of an electro-optically active material; achirped distributed Bragg reflector formed within said electro-opticallyactive material, said chirped distributed Bragg reflector having adirection of propagation; a means for inducing an electric field acrosssaid electro-optically active material in a direction normal to saiddirection of propagation of said chirped distributed Bragg reflector;and, an optical coupling means having an input and an output and aconnection to said waveguide wherein optical signals transmitted intosaid input of said optical coupling means are transmitted into saidwaveguide, said optical signals are reflected back out of said waveguideback into said optical coupling means, and said optical coupling meansprovides said optical signals at said output.
 29. The optical delaygenerator of claim 28 further comprising: a dispersion compensatingfilter, said delay dispersion compensating filter connected to saidoutput of said optical coupling means.
 30. An apparatus for opticalpulse position modulation comprising: a waveguide made of anelectro-optically active material; a chirped distributed Bragg reflectorformed within said electro-optically active material, said chirpeddistributed Bragg reflector having a direction of propagation; amodulating means for applying an electric field across saidelectro-optically active material in a direction normal to saiddirection of propagation of said chirped distributed Bragg reflector;and, an optical coupling means having an input and an output and aconnection to said waveguide wherein optical pulses transmitted intosaid input of said optical coupling means are transmitted into saidwaveguide, said optical pulses are reflected back out of said waveguideback into said optical coupling means, and said optical coupling meansprovides said optical pulses at said output.
 31. The apparatus of claim30 further comprising: a dispersion compensating filter connected tosaid output of said optical coupling means.
 32. A method for opticalpulse position modulation comprising the steps of: generating a seriesof equally spaced optical pulses; transmitting said series of equallyspaced optical pulses into a waveguide comprising electro-opticallyactive material, said waveguide having a chirped distributed Braggreflector formed within said electro-optically active material thatreflects said optical pulses at a turning point within said waveguideand said electro-optically active material having an index ofrefraction, wherein said chirped distributed Bragg reflector generates aseries of delayed optical pulses by reflecting said series of equallyspaced optical pulses at a turning point within said chirped distributedBragg reflector; modulating an electric field applied across saidelectro-optically active material to change said index of refraction ofsaid electro-optically active material so as to change said turningpoint of said chirped distributed Bragg reflector whereby changes insaid turning point result in each pulse within said series of delayedoptical pulses acquiring a delay proportional to changes in said turningpoint; coupling said series of delayed optical pulses out of saidwaveguide.
 33. The method of claim 32 further comprising the step of:transmitting said series of delayed optical pulses into a dispersioncompensating filter.
 34. An optical delay generator comprising: awaveguide made of an electro-optically active material containing achirped distributed Bragg reflector, said chirped distributed Braggreflector having a direction of propagation; a voltage generatorinducing an electric field across said electro-optically active materialin a direction normal to said direction of propagation of said chirpeddistributed Bragg reflector; and, an optical circulator having an inputand an output and a connection to said waveguide wherein optical signalstransmitted into said input of said optical circulator are transmittedinto said waveguide, said optical signals are reflected back out of saidwaveguide back into said optical circulator, and said optical circulatorprovides said optical signals at said output.
 35. The optical delaygenerator of claim 34 further comprising: a dispersion compensatingfilter, said delay dispersion compensating filter connected to saidoutput of said optical circulator.
 36. An apparatus for optical pulseposition modulation comprising: a waveguide made of an electro-opticallyactive material; a chirped distributed Bragg reflector contained in saidelectro-optically active material, said chirped distributed Braggreflector having a direction of propagation; a voltage generator forcontrolling an electric field across said electro-optically activematerial in a direction normal to said direction of propagation of saidchirped distributed Bragg reflector, said voltage generator connected toa data source to be modulated; an optical circulator having an input andan output and a connection to said waveguide wherein optical pulsestransmitted into said input of said optical circulator are transmittedinto said waveguide, said optical pulses are reflected back out of saidwaveguide back into said circulator, and said circulator provides saidoptical pulses at said output.
 37. The apparatus of claim 36 furthercomprising: a dispersion compensating filter connected to said output ofsaid optical coupling means.